Reexamining saline contamination associated with oil and gas development in the Prairie Pothole Region, Sheridan County, MT

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Oil and gas development in the Williston Basin often involves the extraction of saline brines and presents a major source of saline contamination to surface and groundwater resources. The Prairie Pothole Region (PPR) is superimposed over much of the Williston Basin and provides critical habitats for migratory birds and waterfowl. Surface and shallow groundwaters in the PPR often contain high levels of total dissolved solids (TDS); however, salts produced from energy development are chemically different than the near-surface salts. To differentiate between saline contaminated and naturally high TDS water samples, Reiten and Tischmak (1993) developed a Contamination Index (CI = chloride concentration / specific conductance), with contamination indicated by values above 0.035. The Goose Lake study area in Sheridan County, Montana lies within the PPR and has a documented history of saline contamination. Local stratigraphy consists of coarse grained glacial outwash deposits overlying clay-rich glacial till, with extensive contamination in the saturated outwash deposits and wetlands evidenced from 1989 water samples. In 2009, water samples were analyzed from thirty locations including twenty six sites sampled in 1989 to determine changes in chemistry. Additionally, geophysical surveys were conducted to determine stratigraphic controls on contaminant transport using a Geonics EM-31 and EM-34. Widespread contamination was documented in 2009, with one water sample considered uncontaminated based upon the CI data. Two uncontaminated groundwater wells in 1989 had 2009 CI values indicating contamination, documenting the increased aerial extent of contamination due to down-gradient transport. Significant reductions in the median CI values from sites contaminated in 1989 were observed. Assuming reductions in CI values continue at the same rate, contamination would remain for roughly 140 years at the most contaminated site. Geophysical studies showed elevated apparent conductivities associated with saline contamination throughout the study. Contaminant transport is mainly occurring laterally within the outwash deposits, and is evident a minimum of 1,600 m from the source. In contrast, saline penetration into the underlying till is likely less than 30 m. These results demonstrate the potential for co-produced water contamination to aquatic resources far down-gradient of oil and gas developments in glacial outwash deposits within the PPR.